Production method of metallic iron

ABSTRACT

A method producing metallic iron by reducing raw materials including an iron-oxide containing material and a carbonaceous reducing agent under heating, which can minimize re-oxidization of the metallic iron and can efficiently produce metallic iron having a high metallization ratio and high iron purity at high yield. The method produces metallic iron, by heating raw materials including a carbonaceous reducing agent and an iron-oxide containing material in a reduction melting furnace of the moving hearth type, and reducing and melting iron oxides in the raw materials. The reduction melting furnace is partitioned into at least three zones in a hearth moving direction, at least one partitioned zone upstream in the hearth moving direction is a solid-state reducing zone, at least one downstream partitioned zone is a carburization melting zone, and a reduction aging zone is between the solid-state reducing zone and the carburization melting zone.

TECHNICAL FIELD

[0001] The present invention relates to a technique for obtainingmetallic iron with heating reduction of iron oxides, such as iron ore,using a carbonaceous reducing agent, such as coke, and more particularlyto a method capable of producing metallic iron of high purity at a highyield.

BACKGROUND ART

[0002] As a direct iron-making method for obtaining reduced iron throughdirect reduction of an iron-oxide containing material, such as iron oreand iron oxides, with a reducing gas, such as a carbonaceous material,there is conventionally known a shaft furnace process represented by theso-called Midrex process. According to that type of direct iron-makingmethod, a reducing gas produced from natural gas, etc. is blown into ashaft furnace through a tuyere formed in a lower portion of the shaftfurnace, and metallic iron is obtained by reducing iron oxides with theaid of reducing power of the blown reducing gas. Recently, attention hasbeen focused on a process of producing reduced iron, in which acarbonaceous material, such as coal, is used as a reducing agent insteadof natural gas. By way of example, the so-called SL/RN process has beenalready put into practice.

[0003] As another method, U.S. Pat. No. 3,443,931 discloses a process ofmixing a carbonaceous material and powdery iron oxides in the form ofagglomerates or pellets, and reducing the mixture under heating on arotary hearth, thereby producing reduced iron.

[0004] Further, Japanese Unexamined Patent Application Publication No.2000-144224 discloses a method of supplying an iron-oxide materialcontaining a carbonaceous material onto a hearth of a rotary hearthfurnace, and reducing the iron-oxide material under heating, therebyproducing reduced iron. That disclosed method employs a rotaryhearth-furnace having the interior, which is made up of a materialsupply zone (12), burner zones (14, 16), a reaction zone (17), and adischarge zone (18). By employing such a rotary hearth furnace, ironoxides are reduced on the hearth surface kept at high temperature andslag components are separated from produced iron, whereby high-purityiron with a carbon concentration of 1 to 5 mass % can be produced.According to that disclosed method, a highly reducing gas atmosphere ismaintained in the vicinity of raw materials during the progress ofreduction with the presence of a reducing gas (comprising primarilycarbon monoxide) that is generated as a result of the reaction betweenthe carbonaceous material and the iron oxides which are both containedin the raw materials. In the last period of reduction corresponding tothe reaction zone (17), however, the amount of the generated reducinggas is reduced and the concentration of an oxidizing gas, such asmoisture and carbon dioxide generated as exhaust gas upon burnercombustion for heating the raw materials, is relatively increased. Thisleads to a risk that the reduced iron produced as a product isre-oxidized. Particularly, since there is a variation in the progress ofreduction of the iron oxides in the last period of the reduction,reduced iron having progressed more sufficiently in reduction tends tobe more easily re-oxidized. In some cases, therefore, that reduced ironis not sufficiently carburized and melted, and then discharged while itremains in a not-yet molten state.

[0005] In view of the above-mentioned problems in the related art, anobject of the present invention is to establish a technique which isapplied to a process of producing metallic iron by reducing rawmaterials including an iron-oxide containing material and a carbonaceousreducing agent under heating, and which can minimize re-oxidization ofthe metallic iron, i.e., a problem occurred in the last period ofsolid-state reduction, and can efficiently produce metallic iron havinga high metallization ratio and high iron purity at a high yield.

[0006] Another object of the present invention is to establish atechnique which can minimize the FeO concentration in molten slag in thelast period of solid-state reduction, can suppress erosion of a hearthrefractory caused by molten FeO to prolong the life of the hearthrefractory, and is suitably practiced for long-term continuous operationwhile improving maintainability of a plant.

DISCLOSURE OF THE INVENTION

[0007] The present invention having succeeded in overcoming theabove-mentioned problems resides in a method for producing metalliciron, comprising the steps of heating raw materials including acarbonaceous reducing agent and an iron-oxide containing material in areduction melting furnace of the moving hearth type, and reducing andmelting iron oxides in the raw materials, wherein the reduction meltingfurnace is partitioned into at least three zones in a hearth movingdirection, at least one of the partitioned zones on the upstream side inthe hearth moving direction is a solid-state reducing zone, at least oneof the partitioned zones on the downstream side in the hearth movingdirection is a carburization melting zone, and a reduction aging zone isprovided between the solid-state reducing zone and the carburizationmelting zone. When practicing the present invention, preferably, anatmosphere temperature and/or an atmosphere gas composition in thereduction aging zone are adjusted. It is also recommended that anatmosphere modifier be supplied to the reduction aging zone and/or thecarburization melting zone. Further, preferably, the atmosphere modifieris supplied by utilizing a partition wall between the zones. Whenpracticing the present invention, a partition wall between the zones ispreferably provided with one or more openings for communication with theadjacent zone. Further, it is recommended that an atmosphere temperaturein the reduction aging zone is adjusted to the range of 1200 to 1500° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is an explanatory view showing one example of a reductionmelting apparatus used in the present invention.

[0009]FIG. 2 is a sectional view taken along the line A-A in FIG. 1.

[0010]FIG. 3 is an explanatory view showing a section of the apparatusshown in FIG. 1 in the developed form as viewed in the lengthwisedirection of the apparatus.

[0011]FIG. 4 is a photograph showing one example of metallic ironobtained with Example.

[0012]FIG. 5 is a photograph showing one example of metallic ironobtained with Comparative Example.

BEST MODE FOR CARRYING OUT THE INVENTION

[0013] As a result of carrying out intensive studies to solve theabove-mentioned problems occurred in the last period of reduction, i.e.,studies on how to prevent re-oxidization of reduced iron and to preventgeneration of molten FeO, the inventors have found that theabove-mentioned objects can be achieved by providing, in a reductionmelting furnace, an adjusting zone (reduction aging zone) to increase areduction degree of reduced iron produced through a reducing step in thefurnace. In particular, the inventors have accomplished the presentinvention based on the finding that superior effects can be obtained byproperly controlling an atmosphere gas composition and an atmospheretemperature in the reduction aging zone.

[0014] More specifically, according to the present invention, whenproducing metallic iron through the steps of heating raw materialsincluding a material containing an iron-oxide source (hereinafterreferred to also as an iron-oxide containing material), such as iron oreand iron oxides or partly reduced materials thereof, and a carbonaceousreducing agent, such as coke and coal, in a reduction melting furnace ofthe moving hearth type, and then reducing and melting iron oxides in theraw materials, wherein the reduction melting furnace is partitioned intoat least three zones in a hearth moving direction, at least one of thepartitioned zones on the upstream side in the hearth moving direction isa solid-state reducing zone, at least one of the partitioned zones onthe downstream side in the hearth moving direction is a carburizationmelting zone, and a reduction aging zone is provided between thesolid-state reducing zone and the carburization melting zone. Moredetails of the present invention will be described below with referenceto embodiments and the drawings illustrating the embodiments.

[0015] While the following description is made of the case of employing,as raw materials, compacts of raw materials including an iron-oxidecontaining material and a carbonaceous reducing agent (hereinafterreferred to as a “raw-material compacts”), the raw materials used in thepresent invention are not limited to the form of compacts, but may be inthe form of powder. Also, the compact may have any of various shapesincluding the form of a pellet or a briquette.

[0016] FIGS. 1 to 3 are schematic explanatory views showing one exampleof a reduction melting furnace of the moving hearth type developed bythe inventors, to which the present invention is applied. The furnace isof a dome-shaped structure having a rotary moving hearth in the doughnutform. Specifically, FIG. 1 is a schematic perspective view, FIG. 2 is asectional view taken along the line A-A in FIG. 1, and FIG. 3 is aschematic explanatory view showing a section of the reduction meltingfurnace in the developed form as viewed in the rotating direction of therotary hearth in FIG. 1 for easier understanding. In the drawings,numeral 1 denotes a rotary hearth, and 2 denotes a furnace body coveringthe rotary hearth. The rotary hearth 1 is constructed such that it canbe driven by a driving device (not shown) to rotate at a proper speed.

[0017] A plurality of combustion burners 3 are disposed at appropriateplaces in a wall surface of the furnace body 2. Combustion heat andradiation heat generated by the combustion burners 3 are transmitted tothe raw-material compacts on the rotary hearth 1 for performing heatingreduction of the raw-material compacts.

[0018] In a preferred example shown in FIG. 3, the interior of thefurnace body 2 is divided by three partition walls K₁, K₂, K₃ (into asolid-state reducing zone (Z₁), a reduction aging zone (Z₂), acarburization melting zone (Z₃), and a cooling zone (Z₄) in this orderfrom the upstream side in the hearth moving direction). In thatconstruction, those zones are preferably partitioned in such a mannerthat an atmosphere temperature and/or an atmosphere gas composition canbe controlled in each of the zones individually. A charging means 4 forcharging primary raw materials and/or secondary raw materials, such asan atmosphere modifier, is disposed in an opposed relation to the rotaryhearth 1 at the most upstream side in the rotating direction of thefurnace body 2, and a discharging means 6 is provided at the mostdownstream side in the rotating direction (in other words, at the sideimmediately upstream of the charging means 4 because of the rotarystructure). Note that the present invention is not limited to thedivided structure described above, and the furnace structure can be ofcourse changed appropriately depending on the size, target productioncapacity and operation mode of the furnace.

[0019] In operation of such a reduction melting furnace, the rotaryhearth 1 is rotated at a predetermined speed, and the raw-materialcompacts are supplied from the charging means 4 onto the rotary hearth 1such that a layer of the raw-material compacts has a proper thickness.The raw-material compacts charged on the rotary hearth 1 are subjectedto combustion heat and radiation heat generated by the combustionburners 3 while moving in the solid-state reducing zone. Iron oxides inthe compacts are reduced under heating, while maintaining a solid state,with the aid of the carbonaceous reducing agent in the compacts andcarbon monoxide generated upon combustion thereof. Then, reduced ironproduced with almost complete reduction of the iron oxides in thereduction aging zone, described later, is further heated under areducing atmosphere in the carburization melting zone, whereby it iscarburized and melted. The molten iron aggregates into granular metalliciron while separating from slag produced simultaneously. Thereafter, thegranular metallic iron is cooled by any suitable cooling means C in thecooling zone for solidification, and is successively scraped out by thedischarging means 6 provided downstream of the cooling zone.Simultaneously, the slag produced with the granular metallic iron isalso discharged. After passing a hopper H, the metallic iron and theslag are separated from each other with any suitable separating means(such as a sieve-or a magnetic separation device). Finally, metalliciron having iron purity of not less than about 95%, more preferably ofnot less than about 98%, and containing a very small amount of slagcomponents can be obtained.

[0020] While the cooling zone is open to the atmosphere in the structureshown in FIG. 2, it is desired from the practical point of view that thecooling zone be covered to have a substantially enclosed structure forpreventing heat radiation as far as possible and enabling the atmospherein the furnace to be appropriately adjusted.

[0021] The reduction aging zone is a zone that is especially provided inthe present invention to overcome the above-mentioned problems, such asre-oxidization of reduced iron and generation of molten FeO attributableto a variation in progress of reduction of iron oxides in the latterhalf to last period of a conventional reduction process. In other words,the provision of the reduction aging zone is effective in minimizingre-oxidization of reduced iron that has been sufficiently reduced in thesolid-state reducing zone, and in promoting reduction of iron oxidesthat has not been sufficiently reduced. Thus, a variation in reductiondegree among the raw-material compacts can be eliminated, and reducediron having a high reduction ratio (not lower than 80%) can be obtainedin the stage of the reduction aging zone. By carburizing and melting thereduced iron having such a high reduction ratio, metallic iron having ahigh metallization ratio and high iron purity can be efficientlyproduced at a high yield. Also, because the above effect can be moreeasily achieved by adjusting the atmosphere temperature and theatmosphere gas composition in the reduction aging zone, it is preferableto properly adjust them. The method of the present invention will bedescribed below in more detail.

[0022] When heating the raw-material compacts including the carbonaceousreducing agent and the iron-oxide containing material in the reductionmelting furnace of the moving hearth type as described above, if theatmosphere temperature is too high in the solid-state reducing zonewhere iron oxides in the raw materials are reduced, more practically, ifthe atmosphere temperature elevates in excess of the melting point ofslag components, such as a gangue component and not-yet-reduced ironoxides in the raw materials, during a certain period of the reductionprocess, the slag having a low melting point would be melted and wouldreact with a refractory constituting the moving hearth, thereby erodingthe refractory. As a result, a flat and uniform hearth could not bemaintained any more.

[0023] A phenomenon of generation of molten FeO occurred in thesolid-state reducing zone depends on compositions of slag-formingcomponents contained in the carbonaceous reducing agent, the iron-oxidecontaining material, a binder, etc. that constitute the raw-materialcompacts. If the atmosphere temperature in the reduction process exceeds1400° C., the slag having a lower melting point would exude and damagethe hearth refractory in some cases. If it exceeds 1500° C., theundesired melting reduction reaction would progresses regardless of thebrand of iron ore, etc. used as the raw materials, thus resulting inmore noticeable erosion of the hearth refractory. Accordingly, thein-furnace temperature during the reduction process, i.e., thetemperature in the solid-state reducing zone, which is suitable toensure the reduction ratio at a high level without exuding thelower-melting-point slag, is in the range of 1200 to 1500° C. and morepreferably in the range of 1200 to 1400° C. If the in-furnacetemperature is lower than 1200° C., the progress of the solid-statereduction reaction would be slow and the in-furnace retention time hasto be prolonged, thus resulting in poor productivity. On the other hand,if the in-furnace temperature exceeds 1400° C., particularly 1500° C.,exudation of the lower-melting-point slag would occur in the productionprocess regardless of the brand of iron ore, etc. used as the rawmaterials, as described above. Hence, erosion of the hearth refractorywould be noticeable and the operation would be difficult to continue insome cases. A phenomenon of exudation may not occur even in thetemperature range of 1400 or above to. 1500° C. depending on thecomposition and amount of iron ore mixed in the raw materials. However,the frequency and possibility that such a phenomenon does not occur arerelatively low. For those reasons, the temperature in the solid-statereducing zone is preferably in the range of 1200 to 1500° C. and morepreferably in the range of 1200 to 1400° C.

[0024] Further, since a large amount of CO gas and a small amount of CO₂gas are generated in the solid-state reducing zone with the reactionbetween the iron-oxide source and the carbonaceous material in theraw-material compacts charged into the furnace, a satisfactory reducingatmosphere is held in an area in the vicinity of the raw-materialcompacts because of the shielding effect developed by the CO gasgenerated from the raw-material compacts themselves. In the solid-statereducing zone, therefore, adjustment of atmosphere gas conditions is notparticularly required for the reason that a highly reducing atmosphereis maintained by CO gas generated in large amount upon combustion of thecarbonaceous material in the raw-material compacts. However, if thereduction process is performed for a long time in the solid-statereducing zone, the problems, e.g., re-oxidization of reduced iron, ariseas described above. Accordingly, it is desired that the reduced iron betransferred to the reduction aging zone at the time when the reductionratio of the iron oxides in the raw-material compacts reaches a certainhigh value (preferably not lower than 80%).

[0025] If the raw-material compacts in which the solid-state reductionratio of the iron oxides is lower than 80% are heated and melted in thecarburization melting zone, the lower-melting-point slag would exudefrom the raw-material compacts and damage the hearth refractory in somecases, as described above. On the other hand, by heating and melting theraw-material compacts in the carburization melting zone after obtainingthe reduction ratio of not lower than 80%, more preferably not lowerthan 95%, reduction of FeO remaining in a part of the raw-materialcompacts progresses inside the compacts regardless of the brand andamount of iron ore, etc. mixed in the raw-material compacts. As aresult, exudation of the slag can be minimized and stable continuousoperation can be realized without causing erosion of the hearthrefractory.

[0026] As described above, however, the inventors have conductedexperiments and gained the finding as follows. The reduction ratios ofthe raw-material compacts having been reduced in the solid-statereducing zone vary among the raw-material compacts. In particular,reduced iron having progressed more sufficiently in reduction tends tobe more easily re-oxidized and to have a lower reduction ratio.Therefore, that reduced iron is not sufficiently carburized and isharder to melt in the carburization melting process, and then dischargedwhile it remains in a not-yet molten state. Thus, a satisfactory levelof quality cannot be ensured.

[0027] To prevent the reduced iron from being discharged in a not-yetmolten state, temperature has to be elevated in the latter half to lastperiod of the reduction process, and the amount of fuel has to beincreased to maintain an elevated temperature state. Further, theelevation of temperature causes the hearth refractory to more badlydamaged by molten FeO, and hence increases the cost of maintenance suchas repair. Accordingly, the provision of the zone (reduction aging zone)for adjusting the reduction ratio of the raw-material compacts isrequired to prolong the life of the hearth refractory while suppressingerosion of the hearth refractory by the molten FeO, and to progress thesolid-state reduction efficiently and to eliminate a variation inreduction degree while minimizing re-oxidization of reduced iron.

[0028] Also, in the present invention, it is desired to properly controlthe atmosphere temperature and/or the atmosphere gas composition in thereduction aging zone. Proper control of the atmosphere temperatureand/or the atmosphere gas composition in the reduction aging zone iseffective in promoting reduction of not-yet-reduced FeO while preventingmelting of the same, and in preventing re-oxidization of reduced iron.In particular, such control is recommended to efficiently progress thereduction to such an extent that the reduction ratio (oxygen removalratio) is not lower than 80% and preferably not lower than 95%, whilemaintaining, in a solid state, the raw-material compacts charged intothe furnace, without causing partial melting of slag componentscontained in the raw-material compacts.

[0029] The atmosphere temperature in the reduction aging zone is notlimited to a particular value, but the following can also be said aswith the atmosphere temperature in the solid-state reducing zone. If theatmosphere temperature is lower than 1200° C., the progress of thesolid-state reduction reaction would be slow and the in-furnaceretention time has to be prolonged, thus resulting in poor productivity.On the other hand, if the atmosphere temperature exceeds 1400° C.,particularly 1500° C., exudation of the lower-melting-point slag wouldoccur in the production process regardless of the brand of iron ore,etc. used as the raw materials, as described above. Hence, erosion ofthe hearth refractory would be noticeable and the operation would bedifficult to continue in some cases. Accordingly, the temperature in thereduction aging zone is preferably in the range of 1200 to 1500° C. andmore preferably in the range of 1200 to 1400° C. It is then recommendedthat the atmosphere temperature be set to a temperature as high aspossible within the range in which no melting takes place.

[0030] As a matter of course, in the actual operation, it is possible toset the in-furnace temperature in the solid-state reducing zone to benot higher than 1200° C., and to progress the solid-state reduction inthe reduction aging zone at temperature elevated to the range of 1200 to1500° C. Thus, the respective temperatures in those zones may beindividually adjusted as appropriate and set to specific valuesdepending on the corresponding purposes.

[0031] Because the amount of CO gas generated from the raw-materialcompacts transferred to the reduction aging zone is greatly reduced,reduced iron having progressed more sufficiently in reduction generatesCO gas in less amount and the self-shielding action is reduced, asdescribed above. Hence, that reduced iron tends to be more easilyaffected by combustion exhaust gas (oxidizing gases such as CO₂ and H₂O)generated upon burner heating, and metallic iron having been reducedonce tends to be more easily re-oxidized. By adjusting the atmospheregas composition in the reduction aging zone so as to provide a reducingatmosphere, therefore, it is possible to more effectively preventre-oxidization of reduced iron that has been already produced, toprogress reduction of iron oxides that have not yet progressedsufficiently in reduction, and to eliminate a variation in reductiondegree among the raw-material compacts. As a result, reduced iron havinga high reduction ratio (not lower than 80%) can be obtained at a highefficiency.

[0032] The method of adjusting the atmosphere gas composition in thereduction aging zone is not limited to a particular one, but it isdesired that the atmosphere gas composition in the reduction aging zonebe adjusted so as to provide a reducing gas by supplying, for example,an atmosphere modifier. Preferably, the reduction degree CO/(CO+CO₂) ofthe reducing gas in the vicinity of the raw-material compacts is held tobe not less than 0.5.

[0033] As the atmosphere modifier, a carbonaceous material and/or areducing gas is preferably employed. Examples of the carbonaceousmaterial include coals, cokes and so on. When employing coal powder asthe atmosphere modifier, the coal powder is preferably pulverized into agrain size of not greater than 3 mm, more preferably not greater than 2mm, when used. The thus-pulverized coal powder tends to easily unitewith oxygen and generate CO under heating, and is advantageous in moresurely preventing re-oxidization of reduced iron. Further, inconsideration of the yield in supply to the furnace, operability, etc.in the actual operation, the grain size of the coal powder is optimallyin the range of 0.3 to 1.5 mm.

[0034] The method of supplying the atmosphere modifier is not limited toa particular one, and the atmosphere modifier may be supplied toward thehearth by, for example, providing any desired number of atmospheremodifier supply means (not shown) at any suitable positions in thereduction aging zone. In that case, for keeping the atmosphere gascomposition in the vicinity of the raw-material compacts within a rangeto provide a reducing condition, the atmosphere modifier is preferablysupplied to a position as close as to the raw-material compacts. Asanother method of supplying the atmosphere modifier, the atmospheremodifier may be supplied by utilizing the partition wall that partitionsthe reduction aging zone. Utilization of the partition wall can berealized, for example, by providing supply means, such as atmospheremodifier supply pipes, in association with the partition wall K₁ (oneither side facing the solid-state reducing zone or the reduction agingzone, or inside the partition wall), or by assembling those supply pipesin the partition wall. With the method of supplying the atmospheremodifier by utilizing the partition wall, the atmosphere modifier supplypipes can be easily supported and water-cooled, and a change in gas flowcaused by the presence of feed pipes is avoided unlike the case ofproviding the atmosphere modifier supply pipes at any desired positionin the reduction aging zone. Accordingly, radiation heat can beprevented from being unevenly transmitted to the raw-material compactson the hearth. Further, by providing atmosphere modifier supply ports ofthe supply pipes at a level below the partition wall, the atmospheremodifier can be supplied to the vicinity of the raw-material compacts,and can be avoided from being raised up with the gas flow in thefurnace.

[0035] Alternatively, the atmosphere modifier may be laid on the hearthprior to charging of the raw-material compacts, and the thickness of alayer of the atmosphere modifier thus laid is not limited a particularvalue. When laying coal powder, for example, as the atmosphere modifier,the thickness of a layer of the laid coal powder is not limited aparticular value, but the absolute amount of the atmosphere modifierwould be insufficient if the layer thickness is too thin. The layerthickness is preferably not less than about 2 mm and more preferably notless than about 3 mm. Although there is no particular upper limit in thelayer thickness, the atmosphere modifying action is saturated and theatmosphere modifier is economically wasted when the atmosphere modifieris laid in a larger thickness than necessary. From the practical pointof view, therefore, the layer thickness of the atmosphere modifier ispreferably held not more than about 7 mm and more preferably not morethan about 6 mm. The atmosphere modifier may be any other suitablematerial, such as coke or charcoal, other than coal so long as it servesas a CO generating source. As a matter of course, a mixture of thosesuitable materials is also usable.

[0036] By laying the atmosphere modifier on the hearth prior to chargingof the raw-material compacts, the layer of the atmosphere modifier actsto protect the hearth refractory against exudation of molten slag, whichsometimes occurs depending on a variation in operating conditions duringthe reducing and melting steps.

[0037] When employing a reducing gas as the atmosphere modifier, ahydrocarbon-based gas, such as CO, H₂ or CH₄, is preferably employed.Examples of hydrocarbon-based gas include natural gas (particularly gascontaining methane as a main component), a coke furnace gas, and aconverter gas. The method of supplying a reducing gas is not limited toa particular one. As shown in FIG. 3, by way of example, any desirednumber of gas supply nozzles 7 may be provided in the reduction agingzone such that the reducing gas is blown toward the hearth from anydesired positions. As another method, the atmosphere gas may be suppliedby providing supply means, such as reducing gas supply pipes, inassociation with the partition wall K₁ (on either side facing thesolid-state reducing zone or the reduction aging zone, or inside thepartition wall), or by assembling those supply pipes in the partitionwall. With that method, the supply pipes can be easily supported andwater-cooled, and a change in gas flow caused by the presence of feedpipes is avoided. Accordingly, radiation heat can be prevented frombeing unevenly transmitted to the raw-material compacts on the hearth.Further, providing supply ports of the supply pipes at a level below thepartition wall is preferable in that the reducing gas can be supplied tothe vicinity of the raw-material compacts.

[0038] By providing the reduction aging zone, as described above, avariation in the reduction progress of reduced iron after exiting thereduction aging zone can be suppressed and the reduction ratio can beincreased. Therefore, exudation of slag from the raw-material compactsunder heating in the subsequent carburization melting zone can beminimized and stable continuous operation can be realized withoutcausing erosion of the hearth refractory. In particular, by properlyadjusting the atmosphere and the temperature in the reduction agingzone, reduction of not-yet-reduced FeO can be progressed whilepreventing melting of the not-yet-reduced FeO, and re-oxidization ofreduced iron can be more efficiently prevented.

[0039] It is desired, as described above, that the raw-materialcompacts, for which the target reduction ratio has been achieved in thereduction aging zone, be transferred to the carburization melting zoneheated to a predetermined temperature. The temperature in thecarburization melting zone is not limited to a particular value, but itis desirably higher than the atmosphere temperature in the reductionaging zone for the purpose of further heating the raw-material compactsto carburize and melt the compacts. Hence, the atmosphere temperature inthe carburization melting zone is preferably in the range of 1300 to1500° C. and more preferably in the range of 1350 to 1500° C. When theatmosphere temperature in the carburization melting zone is set to,e.g., 1425° C., the internal temperature of the raw-material compactstransferred to the carburization melting zone is gradually elevated, butit drops once because of heat consumption as latent heat incidental tomelting of reduced iron. Thereafter, the internal temperature of theraw-material compacts is elevated again and reaches the settingtemperature of 1425° C. Such a temperature drop point can be regarded asa melting start point. Reduced iron particles are carburized byremaining carbon and CO gas, and their melting points are lowered withthe carburization, whereby the reduced iron particles are quicklymelted. For expediting the melting of the reduced iron particles,therefore, it is desired that an amount of carbon required tosufficiently progress the carburization remain in the reduced ironparticles after the solid-state reduction. The required amount of theremaining carbon depends on the mixing ratio between the iron-oxidecontaining material and the carbonaceous reducing agent used forpreparing the raw-material compacts. In general, however, by selectingan initial amount of the mixed carbonaceous material such that theamount of carbon remaining in the reduced iron (i.e., the amount ofsurplus carbon) in a state where the final reduction ratio in thereduction aging zone reaches almost 100%, namely in a state where themetallization ratio reaches 100%, is not less than 1.5%, the reducediron can be quickly carburized so as to have a lower melting point andcan be quickly melted in the temperature range of 1300 to 1500° C. Theamount of carbon remaining in the reduced iron is less than 1.5%, themelting point of the reduced iron would not be sufficiently loweredbecause of a deficiency in carbon amount necessary for thecarburization, and the temperature for heating and melting the reducediron has to be elevated to a level not lower than 1500° C. in somecases.

[0040] The melting temperature of iron not carburized at all, i.e., pureiron, is 1537° C., and the reduced iron can be melted by heating it totemperature higher than 1537° C. However, it is desired that theoperating temperature in the actual furnace be held at temperature aslow as possible to reduce a thermal load imposed on the hearthrefractory, and in consideration of the melting point of slag producedwith granular metallic iron, the operating temperature be preferablyheld not higher than about 1500° C. More specifically, operatingconditions are desirably controlled such that a temperature rise ofabout 50 to 200° C. is obtained relative to the temperature at the startpoint of the melting process. In other words, for smoothly andefficiently progressing the solid-state reduction and the carburizationmelting, the atmosphere temperature in the carburization melting zone ispreferably set to be 50 to 200° C., more preferably 50 to 150° C.,higher than that in the reduction aging zone.

[0041] In the carburization melting zone, melting and aggregation ofminute reduced iron progresses upon lowering of melting point caused bycarburization of the reduced iron with the presence of carbon remainingin the compacts. In this stage, however, the above-mentionedself-shielding effect is also not sufficiently obtained, and the reducediron tends to easily re-oxidize by oxidizing gases generated with burnercombustion. In the period subsequent to this stage, therefore, it isrecommended that the in-furnace atmosphere gas composition be properlycontrolled so as to provide a reducing atmosphere by any suitable methodincluding the use of the atmosphere modifier described above.Preferably, the reduction degree of the atmosphere gas in the vicinityof the compacts is set to be not less than 0.5.

[0042] The atmosphere modifier can be supplied in a similar manner as inthe case of supplying it to the reduction aging zone (when utilizing thepartition wall to supply the atmosphere modifier, the partition wall K₂can be utilized on either side facing the solid-state reducing zone orthe reduction aging zone).

[0043] In the present invention, the metallization ratio and thereduction ratio are used as indices representing the reduction state ofFeO, and they are defined as follows. The relationship between themetallization ratio and the reduction ratio depends on the brand of ironore, etc. used as the iron-oxide source, but it is expressed as follows.

Metallization ratio=[produced metallic iron/total iron in thecompacts]×100 (%)

Reduction ratio=[oxygen amount removed in reduction process/oxygenamount in iron oxides contained in the raw-material compacts]×100 (%)

[0044] For efficiently progressing a series of steps from thesolid-state reduction to the carburization melting, the atmospheretemperature and the atmosphere gas are preferably controlled in eachstage as described above. Stated otherwise, the temperatures in thesolid-state reducing zone and the reduction aging zone are preferablyheld in the range of 1200 to 1400° C. to prevent generation of moltenFeO due to the melting reduction reaction, as described-above, and thetemperature in the carburization melting zone is preferably held in therange of 1300 to 1500° C. More preferably, the temperature in thereduction aging zone is controlled to be held 50 to 200° C. lower thanthat in the carburization melting zone. In order to control theatmosphere temperature and/or the atmosphere gas composition for each ofthe zones individually, each zone is preferably constructed so as tohave high independency. From a practical point of view, it is desiredthat the spacing between the hearth and the lower end of the partitionwall be as small as possible. Increasing independency of each zone,however, increases the speed of gas flowing from one to another zonewhile passing the small spacing and disturbs the gas flow in thevicinity of the raw-material compacts. This may lead to, for example, adifficulty in introducing the atmosphere modifier to the vicinity of theraw-material compacts, or a difficulty in maintaining a reducingatmosphere in the vicinity of the raw-material compacts. For thosereasons, the partition wall between the zones is preferably providedwith one or more openings for communication with the adjacent zone sothat the gas flow is distributed and the amount of gas passing thespacing between the hearth and the lower end of the partition wall. Inpractice, the shape, number, size and positions of the openings are notlimited to particular ones.

[0045] In the reduction melting furnace of the present invention, burnercombustion is employed to heat the raw-material compacts. In thisconnection, using regenerative burners is preferable in points ofreducing the amount of gas generated after combustion, the amount offuel used, and disturbance of the gas flow in the vicinity of the rawmaterials.

[0046] Further, for improving combustion efficiency, it is preferable toadjust a ratio of fuel to combustion air by, for example, preheating airused for combustion of fuel or employing air having a higher oxygenconcentration as air used for combustion of fuel. When burning fuel atan air-to-fuel ratio not higher than a theoretical one, for example, areducing gas is mixed in gas generated after the burning and thereduction degree of the generated gas is increased. On the other hand,when burning fuel at an air-to-fuel ratio not lower than and close tothe theoretical one, the amount of gas generated after the burning isreduced and disturbance of the gas flow in the vicinity of the rawmaterials is also reduced. Hence, it is recommended to select a properair-to-fuel ratio, taking into account both of the above tendencies.

[0047] From the viewpoint of suppressing disturbance of the gas flow inthe vicinity of the raw materials caused by gas generated with burnercombustion, the burners are preferably installed in an upper portion ofthe furnace, and/or installed to face upward in an upper portion of thefurnace.

[0048] After the completion of carburization, melting and aggregation ofthe reduced iron, the reduction degree of the reducing gas is greatlylowered. At this point in the actual operation process, however,metallic iron having melted and aggregated is almost completelyseparated from slag produced with metallic iron, and hence hardlyaffected by the atmosphere gas. By cooling and solidifying such metalliciron, granular metallic iron of high iron grade can be obtained withhigh efficiency.

[0049] Although the time required for carrying out the process of thepresent invention slightly differs depending on, e.g., compositions ofiron ore and a carbonaceous material contained in the raw-materialcompacts, the solid-state reduction, melting and aggregation of ironoxides can be usually completed in about 10 to 13 minutes.

[0050] The metallic iron obtained with the above-described method of thepresent invention contains substantially no slag components and has veryhigh Fe purity. Accordingly, the thus-obtained metallic iron is suitableas an iron source for use in existing steel-making plants such as anelectric arc furnace and a converter.

EXAMPLES

[0051] The construction and operating advantages of the presentinvention will be described below in detail in connection with Example.It is, however, to be noted that the following Example is not purportedto limit the present invention, and various modifications made withoutdeparting from the purports of the present invention mentioned above andbelow are all involved in the technical scope of the present invention.

Example 1

[0052] Iron ore (having a composition of Fe: 69.2%, SiO₂: 1.8%, andAl₂O₃: 0.5%), coal (fixed carbon: 74.3%, volatile matter: 15.9%, andashes: 9.8%), and a small amount of binder for balling (bentonite) wereevenly mixed with each other and then agglomerated into the form ofpellets with diameters of about 18 mm. Metallic iron was produced usingthe pellets as raw-material compacts. More specifically, the pelletswere charged into the reduction melting furnace of the moving hearthtype, shown in FIGS. 1 to 3, in which solid-state reduction of thepellets was progressed while the atmosphere temperature in thesolid-state reducing zone was controlled to 1310° C. The atmospheretemperature in the subsequent reduction aging zone was also controlledto 1310° C. Thereafter, the pellets were transferred to thecarburization melting zone, in which the atmosphere temperature wascontrolled to 1420° C., for carburization, melting, aggregation, andseparation of slag produced with metallic iron. The metallic iron havingbeen melted, aggregated and almost completely separated from the slagwas transferred to the cooling zone in which it was cooled down to about1000° C. for solidification. Then, the solidified metallic iron wasdischarged out of the furnace using a discharging machine. The timerequired from charging of the raw-material compacts to taking-out of themetallic iron was about 16 minutes. The obtained metallic iron had the Ccontent of 2.6% and the S content of 0.1%. Also, the obtained metalliciron could be easily separated from the slag produced with metalliciron. An external appearance of the finally obtained metallic iron isshown in FIG. 4 (photograph).

[0053] Additionally, a reducing gas (gas mixture containing H₂ of about57% and CH₄ of about 25% as main components) was introduced to each ofthe reduction aging zone and the carburization melting zone throughsupply mechanisms, which were provided respectively in those zones,under adjustment such that the reduction degree CO/(CO+CO₂) of theatmosphere gas composition was held to be not less than 0.5.

Comparative Example 1

[0054] An experiment was conducted in the same manner as in aboveExample 1 except that the reducing gas was not introduced to both thesolid-state reduction aging zone and the carburization melting zone. Asa result, some parts of obtained metallic iron were not yet melted asshown in FIG. 5, and a commercial value of the obtained metallic ironwas poor.

INDUSTRIAL APPLICABILITY

[0055] According to the present invention constructed as describedabove, the reduction aging zone for adjusting the reduction ratio ofreduced iron produced with reduction of iron oxides in the solid-statereduction zone is provided between the solid-state reduction zone andthe carburization melting zone. It is hence possible to progressreduction of the iron oxides, which have not yet sufficiently progressedin reduction, while minimizing re-oxidization of the reduced iron thathas been sufficiently reduced in the solid-state reduction zone, andwhile preventing melting of not-yet-reduced FeO. As a result, avariation in reduction degree among raw-material compacts can beeliminated.

[0056] Also, by properly controlling the atmosphere gas compositionand/or the atmosphere temperature in the reduction aging zone,re-oxidization of the reduced iron can be minimized to increase metalliciron purity, and erosion of a hearth refractory due to exudation ofmolten slag and generation of molten FeO can be minimized. As a result,granular metallic iron of high iron purity can be-efficiently producedwith continuous operation.

1. A method for producing metallic iron, comprising the steps of heatingraw materials including a carbonaceous reducing agent and an iron-oxidecontaining material in a reduction melting furnace of the moving hearthtype, and reducing and melting iron oxides in the raw materials, whereinsaid reduction melting furnace is partitioned into at least three zonesin a hearth moving direction, at least one of the partitioned zones onthe upstream side in the hearth moving direction is a solid-statereducing zone, at least one of the partitioned zones on the downstreamside in the hearth moving direction is a carburization melting zone, anda reduction aging zone is provided between the solid-state reducing zoneand the carburization melting zone.
 2. The producing method according toclaim 1, wherein an atmosphere temperature and/or an atmosphere gascomposition in the reduction aging zone are adjusted.
 3. The producingmethod according to claim 1, wherein an atmosphere modifier is suppliedto the reduction aging zone.
 4. The producing method according to claim1, wherein an atmosphere modifier is supplied to the carburizationmelting zone.
 5. The producing method according to claim 1, wherein theatmosphere modifier is supplied by utilizing a partition wall betweenthe zones.
 6. The producing method according to claim 1, wherein apartition wall between the zones is provided with one or more openingsfor communication with the adjacent zone.
 7. The producing methodaccording to claim 1, wherein an atmosphere temperature in the reductionaging zone is adjusted to the range of 1200-1500° C.